Accepting integers as keys of **kwargs

Could you implement a Python function that can receive integers in the kwargs mapping?

No, you can't. The Python evaluation loop handles calling functions defined in Python code differently from calling a callable object defined in C code. The Python evaluation loop code that handles keyword argument expansion has firmly closed the door on non-string keyword arguments.

But SimpleNamespace is not a Python-defined callable, it is defined entirely in C code. It accepts keyword arguments directly, without any validation, which is why you can pass in a dictionary with non-string keyword arguments.

That's perhaps a bug; you are supposed to use the C-API argument parsing functions, which all do guard against non-string keyword arguments. SimpleNamespace was initially designed just as a container for the sys.implementation data^*, and wasn't really designed for other uses.

There might be other such exceptions, but they'll all be C-defined callables, not Python functions.

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^* The time.get_clock_info() method also runs an instance of the SimpleNamespace class; it's the only other place that the type is currently used.

SimpleNamespace now rejects integer keyword keys. As Martijn supposed, the original behavior was a bug. It seems that it was fixed by bpo-31655: Validate keyword names in SimpleNamespace constructor in v3.9.0b2, and then backported to 3.6.